In dental treatment, a dental prosthesis (such as an inlay, a crown, and an artificial tooth) comprising a material such as a metal having a complicated shape adaptable to each patient is generally used. As one of the methods for casting a metal that have long been conducted in manufacturing not only dental prostheses but also jewelry, fine arts and crafts, parts, or the like having a sophisticated and complicated shape, there is a lost wax process of precision casting. In the case where the prosthesis or the like is manufactured by the lost wax process, a part (object) where a patient is in need for is shaped in the first place using an impression material, and, based on this, a dental gypsum model is made. And a technician precisely manufactures a model the shape of which is the same as the object from the dental gypsum model with wax by hand, the embedding material comprising a refractory material is poured into the surroundings of the obtained wax pattern (wax model) and hardened, and then heating is conducted to form a mold by causing the wax pattern to disappear (to be lost) (incineration or dewaxing). Thereafter, a molten metal is poured into the space of the formed mold, and a cast is taken out by breaking the mold after cooling to obtain the intended prosthesis or the like having a complicated shape.
Amid the times when the image analysis technology has advanced by leaps and bounds, the technology by which a sophisticated image data is obtained by three-dimensionally scanning the gypsum model manufactured in the manner as described above, the prosthesis or the like is digitally portrayed on a PC, and a resin pattern is output as a mechanically precise stereo image using a 3D printer has been developed and utilized in recent years. It is anticipated that the technology spreads with the advance of the image analysis technology in the future. In this case, heating is conducted using the resin pattern that has been output as a stereo image in place of the above-described wax pattern to form a mold by causing the resin pattern to disappear (to be lost), and the subsequent processes are to be conducted.
The embedding materials for use in the above-described technology comprising different materials are used depending on the kind of metal to be cast. Representative examples of the embedding material include “gypsum-based embedding materials”, “phosphate-based embedding materials”, “silica-based embedding materials”, and so on. The “gypsum-based embedding materials” are used for casting a metal having a relatively low melting point. More specifically, in the case where casting is conducted with a metal having a melting point of 1100° C. or less (within a range of noble metal alloy that can be melted with a gas burner) such as, for example, silver (Ag)-based alloy and palladium alloy including gold-palladium alloy (Au—Ag—Pd), the “gypsum-based embedding materials” are used. On the other hand, in the case where casting is conducted with a metal having a melting point of more than 1100° C. such as, for example, cobalt-chromium (Co—Cr)-based alloy and a nickel-chromium (Ni—Cr)-based alloy having a melting point of 1200° C. to 1400° C., the “phosphate-based embedding materials” are used. Among others, although the “gypsum-based embedding materials” are inferior to the “phosphate-based embedding materials” in casting properties at a high temperature, the “gypsum-based embedding materials” have advantages of being excellent in a taking-out property of a cast and operability (fluidity) and providing less deformation due to residual stress and less change with time, and are widely used.
In recent years, the heating process in forming a mold by pouring and hardening the above-described embedding material, and thereafter causing the wax pattern or the like to disappear (to be lost) (incineration or dewaxing) by heating has been changed from the “usual heating” in which the temperature of an electric furnace is gradually raised from room temperature to a target temperature to the “rapid heating” in which a molding material is placed in an furnace having a target temperature to immediately start casting from the standpoint of treatment efficiency. Therefore, the embedding material is required not to cause cracks, breakage, damage, or the like even when subjected to the rapid heating. To meet the requirement, since metals as listed above each have a different coefficient of contraction when solidified, an embedding material having a coefficient of expansion to compensate for the coefficient of contraction of a metal, the embedding material containing cristobalite or quartz is used. On the other hand, there is a problem that the embedding material should be the one that is capable of preventing the occurrence of cracks, breakage, or the like liable to occur by the expansion being too large in order for the embedding material to be applicable to the above-described rapid heating. Moreover, as described previously, since the “gypsum-based embedding materials” in particular have excellent properties but are inferior to the “phosphate-based embedding materials” in casting properties at a high temperature, the “phosphate-based embedding materials” are used in the case where the “rapid heating” is conducted using a resin pattern. Accordingly, when a “gypsum-based embedding material” that is applicable to the “rapid heating” using a resin pattern is provided, the “gypsum-based embedding material” is extremely useful. In addition, the incineration temperature to cause the pattern to disappear in the conventional technology is taken as 700 to 750° C. in the case of using a gypsum-based embedding material and 800 to 900° C. in the case of using a phosphate-based embedding material.
Against the above-described circumstances, proposals as described below have been made in the past. There is a proposal on a gypsum-based embedding material that does not cause cracks, breakage, damage, or the like even when subjected to rapid heating, the gypsum-based embedding material comprising, as main components, for example, calcined gypsum, and cristobalite and quartz each having a particular average particle diameter, to which an inorganic salt and a powdered refractory material having an average particle diameter larger than the above-described average particle diameter of the cristobalite and the quartz are added as components for increasing air permeability (see, Patent Literature 1). Moreover, there is a proposal on a gypsum-based embedding material for casting comprising a heat-insulating material and hemihydrate gypsum, the gypsum-based embedding material being applicable to casting at a high temperature by adding an MgO—Al2O3 spinel as a heat-insulating material (see, Patent Literature 2). Moreover, there is a proposal that, by adding calcium carbonate to main components comprising hemihydrate gypsum and a heat-insulating material, the air permeability is improved and the occurrence of cracks in a mold and burrs in a cast due to generation of a gas through the decomposition of calcined gypsum or wax in calcination at a high temperature are suppressed (see, Patent Literature 3). Moreover, there is a proposal that, by replacing a part of quartz or cristobalite excellent in performance to compensate for the coefficient of contraction of a metal with tridymite in the gypsum-based embedding material or phosphate-based embedding material, the rapid heating of a dental mold is made possible, the time required for the disappearance of a wax pattern and the time required for the preheating of a mold at the time of casting is largely shortened, and casting with high precision is made possible (see, Patent Literature 4). According to the studies made by the present inventors, although the above-described tridymite the rise in the coefficient of thermal expansion of which is calmer when compared with cristobalite, the tridymite is not different from the cristobalite in that it is a heat-expandable refractory material.